Chaac Inc. · Internal Reference

ASHRAE 90.1-2022 Thermal Bridging Compliance Guide

The standard production reference for identifying, quantifying, and modeling thermal bridges on Chaac energy-modeling projects.

SECTION 1

Overview & Chaac Workflow

Path 1 — Our Standard

Table A10.1 Defaults

Use the prescriptive Ψ and χ default values from ASHRAE 90.1-2022 Table A10.1. This is Chaac's default path for nearly all projects — fast, defensible, and code-compliant.

Path 2 — Advanced Only

ISO 10211 Simulation

Numerical 2D/3D heat transfer modeling per ISO 10211 (e.g. LBNL THERM). Reserved for unusual assemblies or when defaults cannot be applied. Requires PM approval before scoping.

Standard 7-Step Workflow

Identify all thermal bridges in the envelope
Classify each by construction type
Determine mitigated vs non-mitigated condition
Quantify lengths (Ψ) and counts (χ) via takeoff
Apply Table A10.1 default factors
Calculate U_eff and integrate into IESVE
Document on COMcheck / compliance forms

Scope Note

ISO 10211 / THERM modeling is advanced scope only and must be quoted separately. Do not commit to simulation work without PM sign-off.

SECTION 2

Psi (Ψ) and Chi (χ) Factors

Linear Thermal Transmittance

Units: Btu/h·ft·°F

Heat loss per unit length along a linear bridge.

Examples

Slab edges
Parapets
Balcony slab projections
Window/door perimeters

Point Thermal Transmittance

Units: Btu/h·°F

Heat loss per discrete point penetration.

Examples

Cladding anchors
Brick ties
Through-insulation fasteners
Structural pin connections

SECTION 3

Step-by-Step Production Workflow

Identify Bridges

Review architectural sections, plans, and details. List every linear and point bridge in the envelope.

Classify Construction Type

Group each bridge by assembly type (mass wall, steel-framed, wood-framed, roof, etc.) per A10.1 categories.

Mitigated vs Non-Mitigated

Determine whether each bridge has a designed thermal break (Schöck, thermally broken bracket, CI continuity, etc.).

Quantity Takeoff

Measure linear length (ft) for Ψ-bridges and count instances for χ-bridges. Document by elevation/floor.

Apply A10.1 Values

Look up the prescriptive Ψ or χ value for each bridge type and condition. Tabulate Q_TB per assembly.

Enter Ψ and χ Values in IESVE

In Construction Manager, open the Thermal Bridges tab for each affected assembly and enter the Ψ value (linear, Btu/h·ft·°F) with its length and the χ value (point, Btu/h·°F) with its instance count. Do not pre-compute U_eff or add a bridge layer to the stack.

COMcheck / Compliance Forms

Calculate U_eff per wall type from the Ψ/χ values and enter into COMcheck. Use the super-wall weighted average only when the project has more than two wall types.

SECTION 4

Table A10.1 Reference & Detail Library

Ψ-Factors (Btu/h·ft·°F) and χ-Factors (Btu/h·°F) per ASHRAE 90.1-2022 Table A10.1. Unmitigated values apply when no thermal break is designed; Default values apply when the prescriptive mitigation in the referenced section is provided. Click any row for the assembly detail viewer.

Class of Construction

Factor Type

Bridge Type	§	Class	Unmitigated		Default (Mitigated)
Bridge Type	§	Class	Ψ	χ	Ψ	χ
Roof edge	5.5.5.1.1	Steel-Framed / Metal	0.450	N/A	0.140	N/A
Parapet	5.5.5.1.2	Steel-Framed / Metal	0.289	N/A	0.151	N/A
Intermediate floor to wall intersection	5.5.5.2.1	Steel-Framed / Metal	0.487	N/A	0.177	N/A
Intermediate floor balcony or overhang to opaque wall intersection	5.5.5.2.2	Steel-Framed / Metal	0.487	N/A	0.177	N/A
Intermediate floor balcony in contact with vertical fenestration	5.5.5.2.2	Steel-Framed / Metal	0.974	N/A	0.177	N/A
Cladding support	5.5.5.3	Steel-Framed / Metal	0.314	N/A	0.217	N/A
Wall to vertical fenestration intersection	5.5.5.4	Steel-Framed / Metal	0.262	N/A	0.112	N/A
Other element and assembly intersections	5.5.5.5	Steel-Framed / Metal	N/A	1.730	N/A	0.910
Roof edge	5.5.5.1.1	Mass (Exterior or Integral)	0.500	N/A	0.100	N/A
Parapet	5.5.5.1.2	Mass (Exterior or Integral)	0.238	N/A	0.125	N/A
Intermediate floor to wall intersection	5.5.5.2.1	Mass (Exterior or Integral)	0.476	N/A	0.179	N/A
Intermediate floor balcony or overhang to opaque wall intersection	5.5.5.2.2	Mass (Exterior or Integral)	0.476	N/A	0.179	N/A
Intermediate floor balcony in contact with vertical fenestration	5.5.5.2.2	Mass (Exterior or Integral)	0.974	N/A	0.177	N/A
Cladding support	5.5.5.3	Mass (Exterior or Integral)	0.270	N/A	0.186	N/A
Wall to vertical fenestration intersection	5.5.5.4	Mass (Exterior or Integral)	0.188	N/A	0.131	N/A
Other element and assembly intersections	5.5.5.5	Mass (Exterior or Integral)	N/A	0.910	N/A	0.190
Roof edge	5.5.5.1.1	Mass (Interior)	0.500	N/A	0.100	N/A
Parapet	5.5.5.1.2	Mass (Interior)	0.511	N/A	0.227	N/A
Intermediate floor to wall intersection	5.5.5.2.1	Mass (Interior)	0.476	N/A	0.286	N/A
Intermediate floor balcony or overhang to opaque wall intersection	5.5.5.2.2	Mass (Interior)	0.476	N/A	0.286	N/A
Intermediate floor balcony in contact with vertical fenestration	5.5.5.2.2	Mass (Interior)	0.974	N/A	0.177	N/A
Cladding support	5.5.5.3	Mass (Interior)	0.270	N/A	0.186	N/A
Wall to vertical fenestration intersection	5.5.5.4	Mass (Interior)	0.313	N/A	0.083	N/A
Other element and assembly intersections	5.5.5.5	Mass (Interior)	N/A	0.910	N/A	0.190
Roof edge	5.5.5.1.1	Wood-Framed / Other	0.450	N/A	0.140	N/A
Parapet	5.5.5.1.2	Wood-Framed / Other	0.032	N/A	0.032	N/A
Intermediate floor to wall intersection	5.5.5.2.1	Wood-Framed / Other	0.336	N/A	0.049	N/A
Cladding support	5.5.5.3	Wood-Framed / Other	0.186	N/A	0.043	N/A
Wall to vertical fenestration intersection	5.5.5.4	Wood-Framed / Other	0.150	N/A	0.099	N/A
Other element and assembly intersections	5.5.5.5	Wood-Framed / Other	N/A	0.330	N/A	0.070

Ψ in Btu/h·ft·°F · χ in Btu/h·°F · Source: ASHRAE 90.1-2022 Table A10.1.

SECTION 5

IESVE Integration

Scope of this Section

In IESVE we apply the Ψ and χ factors directly to each construction. IESVE handles the U-effective adjustment internally — do not pre-compute U_eff and overwrite U_clear in the construction stack.

Compile the Ψ (linear) and χ (point) values for every bridge from Section 4 (A10.1 lookup).

Open Construction Manager in IESVE and select the affected wall/roof assembly.

Open the Thermal Bridges tab for that construction.

Enter each linear bridge as a Ψ-value (Btu/h·ft·°F) with its measured length (ft).

Enter each point bridge as a χ-value (Btu/h·°F) with its instance count.

Repeat for every assembly that has bridges. Apply the same construction to all instances in the model.

Critical Modeling Rule

Do NOT add a separate thermal bridge layer in the construction stack — IESVE already accounts for the bridge contribution when Ψ/χ values are entered in the Thermal Bridges tab. Adding a material layer on top double-counts the heat loss.

SECTION 6

COMcheck & Compliance Forms

Primary Approach: U-effective per Wall Type

For the COMcheck compliance form, calculate U_eff for each wall assembly and enter it as the assembly U-value. One row per construction type, matching the IESVE assemblies one-for-one.

U-effective Formula

U_eff = U_clear + Q_TB / (A_wall × ΔT)

Where Q_TB is the total bridge heat loss (sum of Ψ × L × ΔT and χ × ΔT for every bridge in that wall type), A_wall is the gross opaque wall area, and ΔT is the design temperature difference.

Alternate Approach

Super-Wall Weighted Average

Use the super-wall (area-weighted composite) approach only when a project has more than two distinct wall types, where consolidating into a single weighted U-value materially simplifies the compliance form. For one or two wall types, always prefer the per-wall U_eff approach above.

U_super = [ Σ(U_i × A_i) + Q_TB_total / ΔT ] ÷ A_total

Consistency Requirement

The method selected for COMcheck MUST match the IESVE inputs exactly. Mixing per-assembly U_eff in the model with a super-wall in COMcheck (or vice versa) will fail QA review.

SECTION 7

THERM Workflow (Advanced Only)

Scope Gate

This workflow is advanced scope only. Do not begin THERM modeling without explicit PM approval and a separate scope/fee authorization. THERM is LBNL's free 2D finite-element tool for steady-state heat transfer.

Confirm Advanced Scope

Verify PM has approved THERM modeling for the assembly. Document scope in the project tracker.

Build 2D Geometry

Recreate the cross-section in THERM (LBNL free 2D FEM tool) at true scale, including all material layers.

Assign Material Properties

Apply conductivities from manufacturer data or NFRC defaults. Log every value used.

Set Boundary Conditions

Apply ASHRAE-standard interior/exterior film coefficients and design temperatures.

Run Simulation & Extract Ψ

Solve, then derive Ψ-factor by comparing total heat flow to the 1D clear-field assembly.

Document & Compare

Compile deliverables and compare derived Ψ to the A10.1 default for sanity-check.

Required Deliverables

Deliverable	Description
Psi-factor summary	Tabulated Ψ values for each modeled detail with derivation.
Geometry screenshots	Annotated cross-section views showing material layers.
Isotherm plots	Color isotherm output from THERM showing temperature distribution.
Material property log	All conductivities, densities, and sources used in the model.
Comparison to A10.1	Side-by-side of derived Ψ vs Table A10.1 default with commentary.

SECTION 8

Common Errors & Good Practice

Critical Errors to Avoid

Adding a separate thermal bridge layer in IESVE in addition to U_eff (double counting).
Mixing methods — per-assembly U_eff in IESVE but super-wall in COMcheck (or vice versa).
Applying mitigated Ψ values to assemblies that lack a designed thermal break.
Omitting balcony slabs, parapets, or shelf angles from the takeoff entirely.
Ignoring χ-factor point bridges (anchors, ties, fasteners) — they add up at scale.
Using the wrong ΔT or area when back-solving U_eff, producing nonsensical results.

Good Practice Reminders

Always cross-check derived U_eff against typical ranges before model integration.
Tabulate every Ψ × L and χ count by elevation for traceable QA.
Label every TB-Adjusted assembly clearly in the IESVE Construction Manager.
Save the takeoff spreadsheet, IESVE inputs, and COMcheck form together in the project folder.